Recent developments in the formulation of reinforced thermosetting plastic materials have imparted to the finalized form of such materials qualities of density, hardness, impact resistance, and the like, which are so improved as to provide strong inducement to substitute such materials in many articles heretofore fabricated of metal alloys.
Considerable benefits are attainable, as a result of such substitution. For instance, in the fabrication of sewing machine frames, which were previously made of cast iron or more recently by aluminum or magnesium alloy die casting techniques, considerable cost reduction is made possible by fabrication of modern reinforced thermosetting plastic materials.
While long and continuous experimentation prior to this invention gave clear indication as to certain aspects which would be necessary for attaining cost effective utilization of these new materials, one problem was encountered which prevented the attainment of a commercially acceptable sewing machine frame. This problem involved warpage of an upwardly open channel section, namely, the free arm portion of the sewing machine bed.
Consultation with persons skilled in the art of molding and fabricating plastic materials resulted in the identification of numerous factors each of which had a small influence on such warpage condition. Included among the factors identified by those skilled in the art were temperature and curing time variations; temperature gradients in the mold; variations of the material composition; dimensioning of parts, wall thicknesses and relocation of reinforcing ribs in the molded article; location and arrangement of knockout devices in the mold; and inclusion of non-plastic reinforcing elements in or on the molded article. All of these factors taken together could not, however, provide a cost effective acceptable solution to the bed warpage problem in a sewing machine frame.